I. Field of the Invention
The present invention relates generally to workpiece holders and, more particularly, to a workpiece holder for holding the work during a machining operation.
II. Description of the Prior Art
The machining of airfoils or similar objects that cannot tolerate vibration during the machining process has proven to be a major challenge in the machining art. When conventional machining methods are employed with such workpieces, these workpieces shatter, or are otherwise rendered unusable.
The prior art solution to this problem is depicted in FIG. 2 in which an airfoil 10 is shown as a turbine rotor having a hub 12 adapted to rotate about an axis 14 of rotation. A plurality of turbine blades 16 are secured and extend radially outwardly from the hub 12. The turbine blades 16 are thin and usually brittle, oftentimes constructed of ceramic or similar materials, and must be supported against destructive vibration during conventional machining operations. One standard and conventional machining operation for turbine rotors is to grind the outer tips 18 of the turbine blades 16 to a predetermined radius with respect to the axis of rotation 14 of the airfoil 10.
Still referring to FIG. 2, in order to support the turbine blades 16 against vibration during the machining process, it has been the previous practice to encase all of the turbine blades in a heavy metal alloy 20, known in the art as matrix. The machining operation is then carried out, typically by rotating the airfoil 10 at high speed around the axis of rotation 14 and applying a spinning grinding wheel to the outer tips 18 of the turbine blades 16. After completion of the grinding operation, the matrix 20 is melted and drained away from the airfoil 10 thus leaving the completed airfoil 10.
This previously known use of martrix 20 to hold the blades 16 rigid during the machining operation is disadvantageous in several different respects. First, the use of matrix 20 requires that each part must be individually poured and encased with the matrix 20 prior to the machining operation in addition to being melted away following the completion of the machining operation. These steps of casting and subsequently melting the matrix 20 not only are time consuming but also require expensive tooling for both processes.
A still further disadvantage of the use of matrix is that the added weight of the matrix increases the difficulty in handling the airfoil 10 with the matrix 20. The increased difficulty in handling increases the overall labor costs for the machining process.
A still further disadvantage of using the matrix is that the material cost of matrix is very expensive. Moreover, a portion of the matrix 20 is lost both during the machining operation and also during the melting or matrix recovery operation. Contamination of the matrix during the recovery operation also results in expensive matrix losses.
A final, but substantial disadvantage of the use of matrix is that the matrix became embedded in or loaded on the grinding wheel during the grinding machine operation. As a result of this, a turned finish on the blade tips 18 was previously the best finish obtainable for a grinding operation.